The purpose of the proposed research project will be twofold: (1) To study the mechanism of intercellular communication (beat synchronization) between mouse myocardial cells in culture; and (2) To study how the generation of this phenomenon is related to the biogenesis of the low-resistance intercellular junction, the gap junction or nexus. In order to study the biogenesis, information will first be obtained by ultilizing subcellular fractionarion techniques to isolate a homogeneous preparation of gap junctions from intact rabbit cardiac muscle which will be characterized ultrastructurally and biochemically in order to obtain information about its endogenous components, and their contribution to the phenomenon of communication. simultaneoulsy, the mouse ventricular cardiac muscle will be dissociated, enriched for myocardial cells, and these in turn will be plated out for cell culture investigations. Cell culture studies will be designed to obtain optimal conditions for beat synchronization between myocardial cells, and the beat synchronization will be characterized using thin-section and freeze-fracture techniques. Further, the application of fluorogenic esters coupled with beat synchronization will be used to assay communication between intact myocardial cells. Utilizing information from previous procedures, the relationship of generation of intercellular conduction (best synchronization) to the biogenesis of the gap junction will be studied. The events associated with the initial interaction leading to beat synchronization will be studied by freeze-fracture analysis of the regions of cellular interaction. The recognition events leading to the establishment of junctional interaction will be studied by applying components with known cell surface specificities to the myocardial cell culture prior to interaction. Finally, a plasma membrane fraction will be isolated from non-interacting and interacting cell cultures (myocardial), and the two fractions will be compared biochemically with each other, as well as with the isolated gap junctions from intact tissue.